Conventional Display
Recently, with technological development of LCDs and organic EL displays, there have been provided a number of products that achieve high display performance and high capability of energy saving. Since capable of small-size low-profile fabrication, these LCDs and organic EL displays are widely used as the displays of cellular phones, PDAs (personal Digital Assistants), personal computers, laptop computers (notebook computers), television sets, and the like in particular.
Organic EL devices, components of the organic EL displays, are light emitting devices utilizing organic compounds, of which significant performance improvements have been seen in recent years.
In general, the structures of these organic EL devices are broadly divided, depending on what layer is interposed between the anode and the cathode which are made of transparent electrodes, into the following types:
(1) A single-layer type which has the structure that only a luminescent layer comprising an organic compound is arranged between the anode and the cathode, or symbolically the structure of anode/luminescent layer/cathode;
(2) A double-layer type which has the structure that two layers, a hole transporting layer and a luminescent layer, are formed between the anode and the cathode, or symbolically the structure of anode/hole transporting layer/luminescent layer/cathode;
(3) A triple-layer type which has the triple-layer structure that a hole transporting layer, a luminescent layer, and an electron transporting layer are formed between the anode and the cathode. Symbolically, anode/hole transporting layer/luminescent layer/electron transporting layer/cathode; and
(4) A quadruple-layer type which has the quadruple-layer structure that a hole injection layer, a hole transporting layer, a luminescent layer, and an electron transporting layer are formed between the anode and the cathode. Symbolically, anode/hole injection layer/hole transporting layer/luminescent layer/electron transporting layer/cathode.
When any of the device structures is adopted, holes injected from the anode and electrons injected from the cathode reach the luminescent layer through the hole transporting layer or the electron injection layer, and these holes and electrons recombine in this luminescent layer.
Incidentally, devices referable to as EL devices also include organic EL devices of polymer type utilizing polymer compounds and phosphorescence type light emitting devices utilizing phosphorescence, both of which are under research.
Conventional Organic EL Device
The organic EL devices having the structures of sandwiching an organic luminescent layer between (two) electrodes as described above have been put to intensive research, and heretofore subjected to development for such reasons as follows:    (1) Being fully solid devices and thus easy to handle and fabricate;    (2) Capable of self luminescence, thus requiring no light emitting member;    (3) Having excellent visibility, suitable for displays; and    (4) Easy to render full-color.
Nevertheless, there has been a known problem that organic luminescent layers, being organic materials, are typically difficult to transport electrons and holes through and thus prone to deterioration, easily causing a leak current due to secular changes over a long time of use.
On such a problem, various contrivances have been made heretofore.
For example, the patent document 1 to be mentioned later shows an organic EL device in which an energy difference between the work function of the anode and the ionization energy of the hole transporting layer is reduced for the sake of longer life. The patent document 1 describes that in order to achieve such an object, the anode is made of metal oxide materials that have a work function higher than that of indium tin oxide (ITO) and are conductive. For example, RuOx, MoO3, and V2O5 are mentioned as such conductive metal oxides. Organic EL devices using these metal oxides are disclosed in the patent document 1.
In addition, this patent document 1 proposes anodes of double-layer structures in which thin films made of these conductive metal oxide materials and ITO are laminated for the sake of improved light transmittance (%).
The patent document 2 mentioned below discloses an organic EL device in which metal wires are arranged in connection with the transparent electrodes to reduce the resistances of the transparent electrodes.
The patent document 3 mentioned below discloses an organic EL device in which pieces of metal having a low work function are similarly arranged in connection with the transparent electrodes to reduce the resistances of the transparent electrodes.
The patent document 4 mentioned below discloses an example of an EL device that uses an auxiliary metal film. A special insulative film is arranged on the auxiliary metal film to avoid dielectric breakdown.
The patent document 5 mentioned below discloses an organic EL device which comprises an insulative thin film layer between an electrode and an organic luminescent layer in order to allow long-term use. Specifically, the organic EL device disclosed in this patent document 5 adopts the configuration that an insulative thin film layer made of aluminum nitride, tantalum nitride, or the like is arranged between the anode layer and the organic luminescent layer or between the cathode layer and the organic luminescent layer.
The patent document 6 discloses an organic EL device in which an inorganic material layer containing NiO and at least one additive out of In2O3, ZnO, SnO2, or B, P, C, N, and O, or an inorganic material layer made of Ni1-xO (0.05≦x≦0.5) is formed between an electrode layer and the organic luminescent layer, with the objective of providing a low-cost organic EL device that is free of m-MTDATA, tetraaryldiamine derivatives, and the like.
The patent document 7 mentioned below discloses the technique of fluoridizing an ITO surface to obtain ITO having a work function of 6.2 eV.
“Patent document 1”
Japanese Patent Publication No. 2824411 (Japanese Unexamined Patent Application Publication No. Hei 9-63771)
“Patent document 2”
Japanese Unexamined Patent Application Publication No. Hei 4-82197
“Patent document 3”
Japanese Unexamined Patent Application Publication No. Hei 5-307997
“Patent document 4”
Japanese Patent Publication No. Hei 5-76155
“Patent document 5”
Japanese Unexamined Patent Application Publication No. Hei 8-288069
“Patent document 6”
Japanese Unexamined Patent Application Publication No. Hei 9-260063
“Patent document 7”
Japanese Unexamined Patent Application Publication No. 2000-277256
“Patent document 8”
Japanese Unexamined Patent Application Publication No. Hei 9-63771